To do what he's doing, he has to have an inflated ego

Wildly successful entrepreneurs often have an inflated ego. It comes from being right most of the time, and lucky. It is also almost a requirement, to start and run businesses that become wildly successful. Otherwise folks like Musk would listen to all the folks who say "it can't be done." So yes, Musk has an inflated ego, and yes, Apple may find it convenient to hire folks that couldn't cut it at Tesla Motors.

Yes, but that's not a bad thing

Musk's ego is what tells him that what others think is impossible really isn't. His out-of-the-box ideas for things like the Hyperloop are born from that desire to prove himself. Additionally, I'm sure that he actually has the information to back up his claim. At least SOME of his former employees must have ended up at Apple. It's probably just a stretch to say they couldn't make it at Tesla, as there are a lot of reasons for changing jobs.

All part of the image

Elon Musk is very much the egomaniac he appears to be. This is all a publicity stunt to get attention to his brand. Tesla cars are highly overpriced in comparison to even other electric cars. To associate his brand with edge, Musk makes these outlandish statements to gain attention and monetize the image.

Elon Musk is egotistical and thinks too highly of his own company work ethics

High turnover of staff usually indicates underlying issues in a work environment. Musk's comments were not only arrogant, but he has not provided any viable explanation for why many of his managers have left. Rather than address his own internal issues, he has chosen to publicly attack and belittle another major company because some of his former employees now work there.